Abstract
A search for heavy leptons decaying to a $Z$ boson and an electron or a muon is presented. The search is based on $pp$ collision data taken at $\sqrt{s}=8$ TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. Three high-transverse-momentum electrons or muons are selected, with two of them required to be consistent with originating from a $Z$ boson decay. No significant excess above Standard Model background predictions is observed, and 95% confidence level limits on the production cross section of high-mass trilepton resonances are derived. The results are interpreted in the context of vector-like lepton and type-III seesaw models. For the vector-like lepton model, most heavy lepton mass values in the range 114-176 GeV are excluded. For the type-III seesaw model, most mass values in the range 100-468 GeV are excluded.
Highlights
Background estimationStandard Model processes containing three or more lepton candidates can be classified into two categories
The search is based on pp collision data taken at s = 8 TeV by the ATLAS experiment at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb−1
The largest change in normalization due to the fit is in the 4 category for the Z +e flavour channel, where the fitted ZZ normalization exceeds the prediction by 35%
Summary
The ATLAS detector [21] is a multi-purpose detector covering nearly the full solid angle around the pp interaction region. The ID is enclosed in a superconducting solenoid providing a 2 T axial magnetic field, and performs charged particle tracking for |η| < 2.5. The calorimeter system surrounds the solenoid, and consists of electromagnetic and hadronic components. The hadronic calorimeter (HCAL) uses steel/scintillator tiles in the barrel (|η| < 1.7) and copper/LAr in the endcaps (1.5 < |η| < 3.2). In the forward region (3.1 < |η| < 4.9), electromagnetic and hadronic calorimetry is performed using copper/LAr and tungsten/LAr technology. The first level, implemented in hardware, reduces the event rate to less than 75 kHz using a subset of the detector information. The second and third levels are implemented in software, and reduce the event rate to less than 400 Hz using the full detector information
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